Method and apparatus for forging gear

ABSTRACT

A method for forging a gear according to the present disclosure includes pushing a material into a molding space of a teeth profile die from one side of the teeth profile die for molding external teeth by a punch, and after the pushing, discharging a gear having the external teeth formed thereon from another side of the teeth profile die by the punch. The method further includes providing a material outer diameter constraint die for constraining an outer diameter of a previous material before the material is pushed into the molding space of the teeth profile die on the one side of the teeth profile die, and setting an inner diameter size of the material outer diameter constraint die to be smaller than a large diameter size of the gear set in the teeth profile die.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese patent application No. 2017-163117, filed on Aug. 28, 2017, and Japanese patent application No. 2018-129968, filed on Jul. 9, 2018, the disclosure of which are incorporated herein in its entirety by reference.

BACKGROUND

The present disclosure relates to a method and an apparatus for forging a gear.

A gear having external teeth has been manufactured by forging. An example of a method for forging a gear having external teeth is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2014-217876.

According to the method for forging a gear disclosed in Japanese Unexamined Patent Application Publication No. 2014-217876, a material is first pushed into a molding space of a teeth profile die from one side of a teeth profile die for forming external teeth by a punch. After that, a gear having the external teeth formed thereon is discharged from the other side of the teeth profile die by the punch.

SUMMARY

However, in the method for forging the gear according to the related art as disclosed in Japanese Unexamined Patent Application Publication No. 2014-217876, an outer diameter size of the material is the same as or greater than a large diameter size of a teeth profile of the gear set in the teeth profile die. Hence, there has been a problem that a molding load at the time of forging increases, thereby shortening a life of the teeth profile die.

The present disclosure has been made to solve the above-mentioned problem. An object of the present disclosure is to provide a method and an apparatus for forging a gear that can extend a life of a teeth profile die.

An example aspect of the present disclosure is a method for forging a gear including pushing a material into a molding space of a teeth profile die from one side of the teeth profile die for molding external teeth by a punch, and after the pushing, discharging a gear having the external teeth formed thereon from another side of the teeth profile die by the punch. The method comprises:

providing a material outer diameter constraint die for constraining an outer diameter of a previous material before the material is pushed into the molding space of the teeth profile die on the one side of the teeth profile die; and

setting an inner diameter size of the material outer diameter constraint die to be smaller than a large diameter size of the gear set in the teeth profile die.

An example aspect of the present disclosure is an apparatus for forging a gear including:

a teeth profile die for molding external teeth;

a material outer diameter constraint die provided on one side of the teeth profile die and configured to constrain an outer diameter of a material; and

a punch configured to pushes the material, the outer diameter of which has been constrained by the material outer diameter constraint die, into a molding space of the teeth profile die from the one side of the teeth profile die, and then discharge a gear having the external teeth formed thereon from another side of the teeth profile die.

An inner diameter size of the material outer diameter constraint die is set to be smaller than a large diameter size of a teeth profile of the gear set in the teeth profile die.

The above-described example aspects achieve an effect that can provide a method and an apparatus for forging a gear capable of extending a life of a teeth profile die.

The above and other objects, features and advantages of the present disclosure will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front cross-sectional view schematically showing an example of a method for forging a gear according to a first embodiment;

FIG. 2 is a front view showing a configuration example of an apparatus for forging a gear according to the first embodiment;

FIG. 3 is an enlarged front view showing a configuration example in the vicinity of a teeth profile die and a material outer diameter constraint die of the apparatus for forging a gear according to the first embodiment;

FIG. 4 is a front view showing a configuration example of a material according to the first embodiment;

FIG. 5 is a front view showing a configuration example of a material outer diameter constraint die according to the first embodiment;

FIG. 6 is a plan view showing the configuration example of the material outer diameter constraint die according to the first embodiment;

FIG. 7 is a front view showing a configuration example of a teeth profile die according to the first embodiment;

FIG. 8 is an enlarged front view of a region A in FIG. 7;

FIG. 9 is a plan view showing a configuration example of the teeth profile die according to the first embodiment;

FIG. 10 is a perspective view showing a configuration example of the teeth profile die according to the first embodiment;

FIG. 11 is an enlarged perspective view of a region B in FIG. 10;

FIG. 12 is a graph showing an example of an effect of a method for forging a gear according to the first embodiment;

FIG. 13 is a perspective view showing a configuration example of a material outer diameter constraint die according to the first embodiment;

FIG. 14 is a perspective view showing a configuration example of a gear molded by a method for forging a gear according to the first embodiment as seen from the upper end face;

FIG. 15 is a perspective view showing a configuration example of a gear molded by the method for forging a gear according to the first embodiment as viewed from the lower end face;

FIG. 16 is a perspective view showing a configuration example of a material outer diameter constraint die according to a second embodiment;

FIG. 17 is a front view showing a configuration example of a gear forging forming apparatus according to the second embodiment;

FIG. 18 is an enlarged front view of a region C in FIG. 17;

FIG. 19 is a perspective view showing a configuration example of a gear molded by a method for forging a gear according to the second embodiment viewed from an upper end face; and

FIG. 20 is a perspective view showing a configuration example of a gear molded by the method for forging a gear according to the second embodiment viewed from a lower end face.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the embodiment to be described below, a material to be forged is an annular member having a through hole inside thereof, and external teeth are formed on an outer peripheral surface of this material by forging.

First Embodiment <Outline of Method for Forging Gear According to First Embodiment>

Firstly, an outline of a method for forging a gear according to a first embodiment will be described with reference to FIG. 1. FIG. 1 is a front cross-sectional view schematically showing an example of the method for forging a gear according to the first embodiment. For comparison with the first embodiment, FIG. 1 also shows a method for forging a gear according to the related art.

As shown in FIG. 1, in the related art, a material M is set inside a die 901, is pushed from above the die 901 by a punch 902, and is moved downward. A space inside a lower part of the die 901 is a molding space in which external teeth are formed on the material M. The external teeth are formed on an outer peripheral surface of the material M in the molding space. A gear G on which the external teeth are formed is pushed further from above by the punch 902 and is discharged from below.

In the related art, the outer diameter size of the material M is the same as or greater than a large diameter size of a teeth profile of the gear G set in the die 901. Thus, the gear G is molded from the large material M.

Commonly, the greater the outer diameter size of the material M with respect to the large diameter size of the teeth profile of the gear G, the greater a rate of reduction in a cross section at the time of forging becomes. The rate of reduction in the cross section is calculated by the following equation. The rate of reduction in the cross section indicates difficulty of deformation at the time of drawing (the greater the rate of reduction in the cross section, the more difficult the deformation becomes).

The rate of reduction in the cross section={1−(cross-sectional area of the gear G/cross-sectional area of the material M)}×100 [%]

Before and after the forging, the material M stretches along its entire length in an axial direction according to the formation of the teeth profile because of the principle of constant volume. The greater the rate of reduction in the cross section, the greater the amount of the stretch becomes. Hence, the greater the rate of reduction in the cross section, the greater the molding load at the time of forging due to, for example, an increase in a total amount of movement of the material M. A large molding load causes a large deformation resistance to occur as the die 901 cannot endure such a large molding load, thus shortening a life of the die 901.

To avoid this, the outer diameter size of the material M may be set to be smaller than the large diameter size of the teeth profile of the gear G set in the die 901 to mold the gear G from the smaller material M. In this way, the molding load at the time of forging would be reduced.

However, when the outer diameter size of the material M has expanded to the large diameter size of the teeth profile set in the die 901 before the material M enters the molding space of the die 901 (that is, the material M is upset), it will become the same as the case in which the gear G is forged from the large material M. In such a case, the molding load cannot be reduced after all.

On the other hand, in the first embodiment, the material M is set inside a die 91, is pushed from above the die 91 by a punch 92, and is moved downward. Then, the external teeth are formed on the outer peripheral surface of the material M in the molding space inside the lower part of the die 91, and a gear G having the external teeth formed thereon is discharged from below. In this regard, the first embodiment is similar to the related art.

However, in the first embodiment, a shape of an upper part of the molding space of the die 91 is changed so that, when the outer diameter size of the material M is set to be smaller than the large diameter size of the teeth profile set in the die 91, the material M will not be upset before it enters the molding space of the die 91. More specifically, the upper part of the molding space of the die 91 is shaped such that the outer diameter size of the material M before the material M is pushed into the molding space of the die 91 is constrained so as to be a size smaller than the large diameter size of the teeth profile of the gear G set in the die 91.

In this way, when the outer diameter size of the material M is set to be smaller than the large diameter size of the teeth profile set in the die 91, the material M enters the molding space of the die 91 without being upset, thereby achieving a reduction in the molding load at the time of forging. As a result, the life of the die 91 can be extended.

<Apparatus for Forging Gear According to First Embodiment>

Next, an apparatus for forging a gear used in the method for forging a gear according to the first embodiment will be described with reference to FIGS. 2 and 3. FIG. 2 is a front view showing a configuration example of the apparatus for forging a gear according to the first embodiment. FIG. 3 is an enlarged front view showing a configuration example in the vicinity of a teeth profile die and a material outer diameter constraint die of the apparatus for forging a gear according to the first embodiment.

As shown in FIGS. 2 and 3, an apparatus 1 for forging a gear according to the first embodiment includes an upper unit 10 and a lower unit 20.

The upper unit 10 includes a punch 11.

The punch 11 is a member that is driven downward and pushes the material M set in the material outer diameter constraint die 22 from above. In FIG. 2, for easier understanding of the punch 11, the punch 11 is divided into two at an axis line. In FIG. 2, the left side of the punch 11 is lifted, while the right side of the punch 11 is lowered. However, it should be noted that the actual punch 11 is not divided in this way and instead it is driven to be lifted or lowered integrally.

The lower unit 20 includes a teeth profile die 21, the material outer diameter constraint die 22, a pressure plate 23, a die case 24, a mandrel 25, a discharge member 26, and a discharge port 27.

The pressure plate 23 is an annular member having a through hole inside.

The teeth profile die 21 is an annular member that is disposed above the pressure plate 23 and has a through hole inside. The teeth profile die 21 is an external teeth molding member for molding external teeth on the outer peripheral surface of the material M.

The material outer diameter constraint die 22 is an annular member that is disposed above the teeth profile die 21 and has a through hole inside. The material outer diameter constraint die 22 is a member for constraining the outer diameter of the material M before the material M is pushed into the molding space (a through hole part) inside the teeth profile die 21.

The die case 24 is a member that holds the teeth profile die 21, the material outer diameter constraint die 22, and the pressure plate 23.

The axes of the through holes inside the teeth profile die 21, the material outer diameter constraint die 22, and the pressure plate 23 coincide. Further, the inner diameter size of the pressure plate 23 is set to be equal to or greater than the inner diameter size of the teeth profile die 21. The relation between the inner diameter sizes of the teeth profile die 21 and the material outer diameter constraint die 22 will be described later.

The mandrel 25 is a columnar member disposed in the through holes inside the teeth profile die 21, the material outer diameter constraint die 22, and the pressure plate 23. The mandrel 25 is a member fitted to an inner peripheral surface of the material M and that supports the inner peripheral surface of the material M. The mandrel 25 extends from below the pressure plate 23 to the vicinity of an upper surface of the discharge member 26.

The discharge member 26 is a member disposed below the mandrel 25 and is horizontally driven to the left in the drawing. The discharge member 26 is a member for discharging the gear G having the external teeth from below the pressure plate 23 to the outside of the apparatus 1 for forging. When the gear G is discharged to the outside of the apparatus 1 for forging, the discharge member 26 is horizontally driven so that a hollow part 261 of the discharge member 26 is positioned directly below the mandrel 25. Then, the gear G discharged from below the pressure plate 23 falls into the hollow part 261, slides down an inclined plate 262, and is discharged from the discharge port 27 to the outside of the apparatus 1 for forging.

Hereinafter, a method for forging the gear G using the above-described apparatus 1 for forging the gear will be described.

First, the material M is set from above in the material outer diameter constraint die 22, and the inner peripheral surface of the material M is fitted to the mandrel 25 and is set in the material outer diameter constraint die 22. At this time, a lower end of the material M is positioned near an upper end of the material outer diameter constraint die 22.

In this state, the punch 11 is driven downward to push the material M set in the material outer diameter constraint die 22 from above. Then, the material M is moved downward while the inner peripheral surface thereof is supported by the mandrel 25 and the outer diameter size thereof is constrained by the material outer diameter constraint die 22, and then enters the molding space (a through hole part) inside the teeth profile die 21.

The material M having entered the teeth profile die 21 is pushed further downward by the punch 11, and external teeth are formed on the outer peripheral surface of the material M by the teeth profile die 21 during a process of the material M being moved downward in the molding space inside the teeth profile die 21. The gear G having the external teeth formed thereon by the teeth profile die 21 is pushed further downward by the punch 11 and is discharged from below the pressure plate 23.

Here, when the material M is pushed downward by a predetermined distance (roughly, a length of the material M in the axial direction) by the punch 11, the punch 11 is temporarily driven to be lifted. Next, a new material M is set on the previous material M, the punch 11 is driven downward again, and the newly set material M is pushed from above. After that, this operation is repeated. As a result, it is possible to continuously form the gears G on which the external teeth are formed.

As described above, the gear G discharged from below the pressure plate 23 falls into the hollow part 261 that has moved directly under the mandrel 25, slides down the inclined plate 262, and is discharged from the discharge port 27 to the outside of the apparatus 1 for forging.

<Material, Teeth profile die, and Material Outer Diameter Constraint Die According to First Embodiment>

Next, diameter sizes of the material M, the teeth profile die 21, and the material outer diameter constraint die 22 according to the first embodiment will be described with reference to FIGS. 4 to 11.

FIG. 4 is a front view showing a configuration example of the material M according to the first embodiment.

As shown in FIG. 4, the material M is an annular member having a through hole inside. D1 represents the outer diameter size of the material M.

FIGS. 5 and 6 are views showing a configuration example of the material outer diameter constraint die 22 according to the first embodiment. FIG. 5 is a front view, and FIG. 6 is a plan view.

As shown in FIGS. 5 and 6, the material outer diameter constraint die 22 is an annular member having a through hole inside. D2 represents the inner diameter size of the material M. In order to set the material M in the through hole inside the material outer diameter constraint die 22, the inner diameter size D2 of the material outer diameter constraint die 22 is set to be slightly greater than the outer diameter size D1 of the material M. That is, D1<D2.

FIGS. 7 to 11 are views showing a configuration example of the teeth profile die 21 according to the first embodiment. FIG. 7 is a front view, FIG. 8 is an enlarged front view of a region A in FIG. 7, FIG. 9 is a plan view, FIG. 10 is a perspective view, and FIG. 11 is an enlarged perspective view of a region B in FIG. 10.

As shown in FIGS. 7 to 11, the teeth profile die 21 is an annular member having a through hole inside.

The part of the outer peripheral surface of the material M that passes through a region b of the teeth profile die 21 in FIG. 11 becomes a large diameter part of the teeth profile of the gear G. An inner diameter size of the region b in FIG. 11 corresponds to D3 in FIG. 8. Thus, this D3 is the large diameter size of the teeth profile of the gear G set in the teeth profile die 21. The outer diameter size D1 of the material M and the inner diameter size D2 of the material outer diameter constraint die 22 are set to be smaller than the large diameter size D3 of the teeth profile set in the teeth profile die 21. That is, D1<D2<D3.

Then, the material M having the outer diameter size D1 smaller than the large diameter size D3 of the teeth profile set in the teeth profile die 21 is constrained to D2 or less by the material outer diameter constraint die 22 having the inner diameter size D2 smaller than the large diameter size D3 of the teeth profile before the material M is pushed into the teeth profile die 21. Consequently, the material M is not expanded to the large diameter size D3 of the teeth profile (i.e., the material M is not upset), and instead enters the molding space of the teeth profile die 21. This reduces the molding load at the time of forging. As a result, the life of the teeth profile die 21 can be extended.

<Effect of Method for Forging Gear According to First Embodiment>

Next, an effect of the method for forging a gear according to the first embodiment will be described with reference to FIG. 12. FIG. 12 is a graph showing an example of the effect of the method for forging a gear according to the first embodiment. In FIG. 12, the horizontal axis represents the rate of reduction in the cross section [%] at the time of forging, and the vertical axis represents the molding load [Ton] at the time of forging.

In FIG. 12, the solid line shows the relation between the rate of reduction in the cross section and the molding load. Here, one point on the right side represents a result of forging the gear G from the material M having the outer diameter size greater than the large diameter size of the teeth profile of the gear G set in the teeth profile die 21 in accordance with the method for forging a gear according to the related art.

The four points on the left side represent the results of forging the gear G from the material M having the outer diameter size smaller than the large diameter size of the teeth profile of the gear G set in the teeth profile die 21 in accordance with the method for forging a gear according to the first embodiment.

As shown in FIG. 12, in the related art, the gear G is forged from the material M having a large outer diameter size. Thus, the rate of reduction in the cross section is large, and consequently the molding load becomes large. In the related art, when the rate of reduction in the cross section becomes smaller, the molding load would also become smaller. However, as described above, in the related art, even when the outer diameter size of the material M is made smaller to thereby reduce the rate of reduction in the cross section, the material M is upset before the material M enters the teeth profile die 21. Thus, the molding load cannot be reduced.

In contrast, in the first embodiment, the outer diameter of the material M is constrained by the material outer diameter constraint die 22 before the material M is pushed into the teeth profile die 21. Thus, the material M will not be upset. Therefore, by reducing the outer diameter size of the material M and reducing the rate of reduction in the cross section, the molding load can be reduced. As a result, the life of the teeth profile die 21 can be extended.

In the first embodiment, the smaller the rate of reduction in the cross section, the smaller the molding load becomes. Therefore, it is more preferable that the rate of reduction in the cross section be small in terms of reducing the molding load. However, as the rate of reduction in the cross section is made smaller, an unfilled amount of the outer diameter increases. The unfilled amount of the outer diameter represents a difference between the large diameter size of the teeth profile of the gear G set in the teeth profile die 21 (corresponding to the above D3) and the actual large diameter size of the teeth profile of the gear G discharged from the teeth profile die 21. When the unfilled amount of the outer diameter increases, the unfilled amount of the outer diameter of the gear G as a product may fall outside an allowable range of a tolerance. For this reason, the rate of reduction in the cross section is preferably set within a proper range in consideration of both the molding load and the unfilled amount of the outer diameter. In FIG. 12, the two points on the right side in which the rate of reduction in the cross section is high among the four points on the left side according to the first embodiment fall within an allowable range of a tolerance of the unfilled amount of the outer diameter. Therefore, these two points should be decided as the rate of reduction in the cross section.

Moreover, it is preferable that, as described above, the outer diameter size D1 of the material M and the inner diameter size D2 of the material outer diameter constraint die 22 not only define an upper limit but also define a lower value in order to make the rate of reduction in the cross section fall within the proper range. When the pitch circle diameter size of the gear G set in the teeth profile die 21 is denoted by D4, D1 and D2 are preferably greater than the pitch circle diameter size D4 of the gear G. That is, D4<D1<D2<D3.

Second Embodiment <Outline of Method for Forging Gear According to Second Embodiment>

As described above, in the related art, the outer diameter size of the material M is the same as or greater than the large diameter size of the teeth profile of the gear G set in the teeth profile die 21. Thus, the gear G is molded from the large material M. For this reason, a sink mark is not generated at teeth tips of the gear G, and there was no problem concerning the smoothness in the teeth tips of the gear G.

In contrast, in the first embodiment, the outer diameter size of the material M is smaller than the large diameter size of the teeth profile of the gear G set in the teeth profile die 21. Thus, the gear G is molded from the small material M. Further, as shown in FIG. 13, the material outer diameter constraint die 22 according to the first embodiment is a simple annular member with no processing in particular performed on the inner peripheral surface thereof. Hence, at the time of forging, a material of the material M starts to be divided toward a teeth tip direction and a teeth base direction at the same time at a boundary part between the teeth profile die 21 and the material outer diameter constraint die 22, which is a material teeth profile deformation starting part where the teeth profile of the material M starts to deform. This may cause a sink mark to be generated at the teeth tips of the gear G.

FIGS. 14 and 15 are perspective views showing configuration examples of the gear G molded by the method for forging a gear according to the first embodiment. FIG. 14 is a diagram showing the gear G viewed from an upper end face. FIG. 15 is a diagram showing the gear G viewed from a lower end face.

It can be seen that sink marks are generated at the teeth tips of the gear G shown in FIGS. 14 and 15. Thus, the method for forging a gear according to the first embodiment has room for improvement regarding the smoothness in the teeth tips of the gear G.

A method for forging a gear according to a second embodiment is to improve the smoothness in the teeth tips of the gear G. In the second embodiment, the material outer diameter constraint die 22 according to the first embodiment is replaced by a material outer diameter constraint die 22A.

FIG. 16 is a perspective view showing a configuration example of the material outer diameter constraint die 22A according to the second embodiment.

As shown in FIG. 16, a teeth profile shape of the teeth profile die 21 is carved on the inner peripheral surface of the material outer diameter constraint die 22A. In this case, like the first embodiment, an inner diameter size of the material outer diameter constraint die 22A is smaller than the large diameter size of the teeth profile of the gear G set in the teeth profile die 21, and is greater than a pitch circle diameter size of the gear G set in the teeth profile die 21. Thus, the teeth profile shape of the teeth profile die 21 is carved on the inner peripheral surface of the material outer diameter constraint die 22A at an outer side in a radial direction than an inner diameter of the material outer diameter constraint die 22A.

In this manner, the teeth profile shape of the teeth profile die 21 is carved on the inner peripheral surface of the material outer diameter constraint die 22A. Thus, at the time of forging, the material of the material M is already accumulated at the teeth tips on the material outer diameter constraint die 22A side at the material teeth profile deformation starting part (details thereof will be described later). For this reason, unlike the first embodiment, the material of the material M is not divided and can be moved to the teeth tip part on the teeth profile die 21 side. This prevents a sink mark from being generated at the teeth tips of the gear G, thereby resulting in improving the smoothness of the teeth tips of the gear G.

<Apparatus for Forging Gear According to Second Embodiment>

Next, an apparatus for forging a gear used in the method for forging a gear according to the second embodiment will be described with reference to FIGS. 17 and 18. FIG. 17 is a front view showing a configuration example of the apparatus for forging a gear according to the second embodiment. FIG. 18 is an enlarged front view of a region C of FIG. 17. In FIG. 18, a mandrel 25 is not shown in order to make an inner diameter shape of the material outer diameter constraint die 22A and the teeth profile die 21 easier to see.

As shown in FIGS. 17 and 18, an apparatus for forging a gear lA according to the second embodiment differs from the apparatus for forging a gear 1 according to the first embodiment in that in the apparatus for forging a gear 1A, the material outer diameter constraint die 22 of the apparatus for forging a gear 1 is replaced by the material outer diameter constraint die 22A. The configuration of the apparatus for forging a gear 1A other than the material outer diameter constraint die 22A is the same as that of the apparatus for forging a gear 1.

As described above, the material outer diameter constraint die 22A has the teeth profile shape of the teeth profile die 21 carved on the inner peripheral surface thereof at the outer side in the radial direction than the inner diameter of the material outer diameter constraint die 22A (like the first embodiment, the inner diameter size is D2). Consequently, the teeth tips of the gear G appear to have been carved on the inner peripheral surface of the material outer diameter constraint die 22A.

<Effect of Method for Forging Gear According to Second Embodiment>

As described above, in the second embodiment, the teeth profile shape of the teeth profile die 21 is carved on the inner peripheral surface of the material outer diameter constraint die 22A at the outer side in the radial direction than the inner diameter of the material outer diameter constraint die 22A. Thus, at the time of forging, the material of the material M protrudes (upset-moves) toward the teeth tip direction of the carved part of the inner peripheral surface of the material outer diameter constraint die 22A due to an internal pressure applied to the material M while the material M moves through the material outer diameter constraint die 22A. Hence, at the time of forging, the material of the material M is already accumulated at the teeth tips on the material outer diameter constraint die 22A side at the material teeth profile deformation starting part. The material of the material M can be moved to the teeth tips on the teeth profile die 21 side without being divided like in the first embodiment, thereby preventing a sink mark from being generated at the teeth tips of the gear G.

FIGS. 19 and 20 are perspective views showing configuration examples of the gear G molded by the method for forging a gear according to the second embodiment. FIG. 19 is a diagram showing the gear G viewed from an upper end face. FIG. 20 is a diagram showing the gear G viewed from a lower end face.

It can be seen that no sink mark is generated at the teeth tips of the gear G shown in FIGS. 19 and 20. Therefore, the method for forging a gear according to the second embodiment can prevent a sink mark from being generated at the teeth tips of the gear G, thereby improving the smoothness of the teeth tips of the gear G. Effects other than this effect are the same as those in the first embodiment.

Note that the present disclosure is not limited to the above-described embodiment, and can be appropriately changed without departing from the spirit of the disclosure.

From the disclosure thus described, it will be obvious that the embodiments of the disclosure may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims. 

What is claimed is:
 1. A method for forging a gear including pushing a material into a molding space of a teeth profile die from one side of the teeth profile die for molding external teeth by a punch, and after the pushing, discharging a gear having the external teeth formed thereon from another side of the teeth profile die by the punch, the method comprising: providing a material outer diameter constraint die for constraining an outer diameter of a previous material before the material is pushed into the molding space of the teeth profile die on the one side of the teeth profile die; and setting an inner diameter size of the material outer diameter constraint die to be smaller than a large diameter size of the gear set in the teeth profile die.
 2. The method for forging a gear according to claim 1, wherein the inner diameter size of the material outer diameter constraint die is set to be greater than a pitch circle diameter size of the gear set in the teeth profile die.
 3. The method for forging a gear according to claim 1, wherein a teeth profile shape of the teeth profile die is carved on an inner peripheral surface of the material outer diameter constraint die at an outer side in a radial direction than the inner diameter size of the material outer diameter constraint die.
 4. An apparatus for forging a gear comprising: a teeth profile die for molding external teeth; a material outer diameter constraint die provided on one side of the teeth profile die and configured to constrain an outer diameter of a material; and a punch configured to push the material, the outer diameter of which has been constrained by the material outer diameter constraint die, into a molding space of the teeth profile die from the one side of the teeth profile die, and then discharge a gear having the external teeth formed thereon from another side of the teeth profile die, wherein an inner diameter size of the material outer diameter constraint die is set to be smaller than a large diameter size of a teeth profile of the gear set in the teeth profile die.
 5. The apparatus for forging a gear according to claim 4, wherein the inner diameter size of the material outer diameter constraint die is set to be greater than a pitch circle diameter size of the gear set in the teeth profile die.
 6. The apparatus for forging a gear according to claim 4, wherein a teeth profile shape of the teeth profile die is carved on an inner peripheral surface of the material outer diameter constraint die at an outer side in a radial direction than the inner diameter size of the material outer diameter constraint die. 